The present invention deals with an amplifier. More particularly, the present invention deals with a high breakdown voltage amplifier which has low input capacitance.
Magnetic disk drives often have a read/write head which is used for both writing data to a magnetic disk in the magnetic disk drive and reading data from the magnetic disk. During a write operation, a write signal is provided to the read/write head from a write control circuit. The write signal represents data to be written to, or encoded on, the magnetic disk.
During a read operation, the read/write head senses flux reversals from the magnetic disk. The flux reversals represent data encoded on the magnetic disk during a write operation. Based on the flux reversals, the read/write head provides a read signal to a read circuit. The read signal is representative of the data encoded on the magnetic disk. The read circuit amplifies the read signal and recovers the data. The read circuit then provides the data to a magnetic disk controller for further processing.
In a magnetic disk drive in which the read/write head is used to both write data to the disk and read data from the disk, the read/write head is coupled, at all times, to the write circuitry and to the read circuitry. This configuration can cause several problems. For example, during a write operation, it is desirable to achieve a large voltage swing across the read/write head. The large voltage swing ensures that currents switch quickly through the read/write head. This is important because a typical inductive read/write head provides a high inductive load to the write circuit.
However, the read/write head is not only coupled to the write circuitry. Rather, it is also coupled to the read circuitry. Typically, the first circuit to which the read/write head is coupled in the read circuitry is a read amplifier which amplifies the read signal provided by the read/write head. The read amplifier is typically a standard differential common emitter amplifier. Therefore, when the large voltage swings are applied to the read/write head during a write operation, those same voltage swings are also applied to the inputs of the read amplifier in the read circuit. Such large voltage swings can cause the input transistors in the read amplifier to break down in the reverse direction. Such a breakdown causes the beta of the transistors to degrade. Hence, noise increases and the gain of the amplifier is reduced over time.
In the past, pn junction diodes have been used in an effort to solve the problem. The pn junction diodes were placed in the emitter legs of the read amplifier to increase the breakdown voltage required for the transistors to break down in the reverse direction. However, the reverse breakdown voltage of a pn junction diode is only typically about six volts. Thus, even with the pn junction diodes incorporated in the amplifier, it was still difficult to apply the large voltage swings without damaging the read amplifier.
It is also important in the operation of the read and write circuitry that the read amplifier have a low input capacitance. Even with the pn junction diodes in the emitter legs of the read amplifier, the input capacitance of the read amplifier is still relatively high.